Light emitting inter-band semiconductor devices emit light when an energetic electron in the conduction band recombines with a hole in the valence band. Recombination of the electron and hole results in a release of energy in the form of emission of a photon. The smallest difference in energy between the conduction band and valence band, known as the band gap energy Eg, broadly dictates the amount of energy released on recombination of the electron and hole. This amount of energy in turn dictates the wavelength of the photon emitted. Different semiconductor materials have different band gap energy Eg, and the material used to form an active region of a light emitting semiconductor device is therefore selected primarily according to the wavelength of light it is desired that the device emits.
Materials used to form the active region of light emitting semiconductor devices that emit light at IR wavelengths typically comprise group III-V materials, as these tend to have suitable band gaps for emission at IR wavelengths. A problem with such materials is that their properties are strongly dependent on temperature.
Taking a semiconductor laser as an example, there is a threshold current Ith at which the emission of coherent light by the laser begins and a slope efficiency representing the change in power of the optical output in relation to the change in power of the electrical input. Both the threshold current Ith and the slope efficiency are sensitive to temperature, and usually particularly so at and around room temperature. For example, FIG. 1 shows the relationship between the optical power of light output by a conventional 1.5 μm InGaAsP laser and electrical current input to the laser across a range of temperatures. It can be seen that the threshold current Ith, which is the current at which the optical power abruptly increases, increases with increasing temperature. It can also be seen that, for a constant bias current the optical power decreases significantly with increasing temperature. The relationship between power and temperature at a fixed bias current is shown in FIG. 2.
Referring to FIG. 3, it can be seen that at low, e.g. cryogenic, temperatures the threshold current Ith of the 1.5 μm InGaAsP laser is almost entirely due to light producing radiative recombination (the radiative current) which increases linearly with increasing temperature. However, above approximately 260K the threshold current Ith increases strongly. The increase is exponential and therefore wide variations in threshold current Ith occur in the temperature range of 293K to 353K (approximately 20° C. to 80° C.), which is the ambient temperature range for most applications.
In order to mitigate the difficulties arising from the temperature dependence of such semiconductor lasers, thermo-electric control (TEC) is used to maintain a constant temperature and stabilise the threshold current. However, this adds considerable expense to the cost of a semiconductor laser and dramatically increases energy usage. The 1.5 μm InGaAsP laser typically requires around 10 mW of electrical power to achieve laser threshold. However, it will typically require approximately 600 mW of electrical power for the TEC. Therefore only around 2% of the inputted electrical power is used to produce laser emission.